The present invention relates to a single sideband, amplitude modulated transmitter of the type requiring an output stage that linearly amplifies the controlling carrier frequency single sideband. For this purpose, for example, a transmitting tube used as the output stage should actually operate in the class B mode and have a linear Ug.sub.1 /Ia characteristic curve.
Since it is desirable to reduce the power loss occurring in such output stage, the proposal has already been made, as disclosed in German DE-OS 1,766,586 and corresponding U.S. Pat. No. 3,413,570, to modulate the output tube anode voltage at the same time so that such voltage is reduced when there is a small modulation at the control grid. It has also been attempted to avoid the then occurring distortions, as disclosed in U.S. Pat. No. 3,486,128, by varying the collector voltage of a transistor (corresponding to the aforementioned anode voltage comodulation) in accordance with the difference between the envelope curves at the output, on the one hand, and at the input, on the other hand, of the output stage. The effect of such envelope feedback, however, has been found to be insufficient because the control circuit gain cannot be driven to the actually required degree because of the tendency to oscillate.
For audio frequency amplifiers an additional way to reduce power loss is disclosed in lehrbuch der Funkempfangstechnik I [Radio Receiving Textbook], 3rd edition, 1959, by Dipl.-Ing. helmut Pitsch, at Section 292. This publication relates to the class AB amplifier in which the operating point for small modulations is placed so as to produce a smaller anode current than for class A amplifiers so that the power loss will be less than in the class A amplifier with a still relatively low distortion factor for small modulations. With greater modulations, the operating point shifts in the direction toward class B operation due to a drop in the D.C. voltage occurring during the modulation across one cathode resistor as a result of a rectified current due to a partial rectification which occurs in class B operation. The D.C. voltage drop across the cathode resistor is utilized as a negative grid bias to shift the operating point. Since in class B operation the negative half-wave is suppressed, or clipped, for large modulations, such a class AB amplifier must be operated in a push-pull arrangement in order to keep the distortion within limits. To improve the remaining distortion factor, a relatively long time constant of 0.15 second is proposed for the parallel connection of the cathode resistor (e.g. 200 ohms) with a bridging capacitor (of e.g. 750 uF).
This known class AB audio frequency amplifier, compared to a class A amplifier, is intended mainly to reduce the power loss by shifting the operating point in the direction toward class B operation with large modulations with a delayed return to class A operation during small modulations, the delay being effected by means of the cathode resistor/capacitor combination. A worsening of the distortion factor is intentionally taken into consideration here and is reduced again by the push-pull operation and by special design rules for the cathode resistor/capacitor combination. Such a circuit cannot be used for the output stage of a single sideband transmitter for several reasons:
First, a push-pull arrangement is too expensive for a transmitter output stage.
Additionally, although there exist special single sideband high frequency tubes for class B operation which have a grid voltage/anode current characteristic curve approximating a straight line as much as possible, the use of such a tube brings about distortions in the envelope curve of the amplified single sideband signal of such a magnitude that no further distortions can be accepted in connection with the effort to reduce power loss. This applies particularly to the use of an output tube having a curved characteristic that has not been approximately linearized especially for single sideband operation.
The method of reducing power losses by using a class AB amplifier is therefore unsuitable for a single sideband transmitter because it would always bring about additional distortions. It would also not be possible to eliminate these distortions again by special selection of the cathode resistor/capacitor combination since this combination in particular produces additional distortions during the amplification of the single sideband in that the shift of the operating point does not occur in synchronism with the change in RF amplitude.